In a conference call with reporters, Boeing officials acknowledged that they knew of problems with the lap joints that bind together the fuselage of the plane, but they didn't expect it to be an issue until the 737 jets had been through many more takeoff and landing cycles than the plane that got into trouble Friday.

"Our plan was to recommend inspections at 60,000 cycles. Obviously, none are close to that at this point in time," said Paul Richter, the chief project engineer for Boeing's 737 classic. "It is regrettable that we had to accelerate our plans based on an event of this nature."

The 15-year-old Southwest Airlines jet, which tore open along its lap joint last Friday, had 39,000 takeoff and landing cycles.

Boeing's 737 classics were manufactured from 1984 until 2000. In 1993, Boeing redesigned its 737 due to problems with the lap-joint design, but the problem Southwest jet was manufactured after that redesign.

Boeing will now recommend that all 737 jets that cross the 30,000-cycle threshold be inspected for fatigue cracks. The inspections are to be repeated after every 500 flight cycles.

The number of affected jets now stands at 175, though that will eventually climb to over 500 jets as the 737s age and run up more flight cycles.

Affected jets are used by airlines around the world. Eighty of the planes are in service in the United States, most of them for Southwest.

Boeing said that the newer 737 currently in production has a different lap-joint design and should not experience cracking.

Boeing's admission came just hours before the Federal Aviation Administration formally ordered emergency inspections on those 175 jets. The government is now conducting a wholesale reassessment of its approach to plane inspections.

Inspections will initially focus on 737's that have accumulated or are close to accumulating 30,000 flight cycles. Inspections must be conducted within 20 days, and even sooner -- within five days -- for planes with more than 35,000 cycles. If cracks are found, repairs must be completed before a jet can return to flight.

737 Cracks: Boeing Knew of Problem, But Only in Older Jets

Inspectors use something called eddy current technology to examine the 50-foot long lap joint, passing an electric current through an aircraft's skin to look for small cracks. If any warning signs are detected, ultrasound and X-ray tools are then used for a closer examination. In some areas, a plane's skin can be as thin as a nickel.

Southwest said it resumed its regular schedule today. It said this morning that it had finished inspections of all 79 of the 737-300s it removed from service after the weekend. The airline said it uncovered fatigue cracks in five planes, and would follow Boeing's advice on how to repair them. Southwest said it believes its inspection process complies with the government's order.

Southwest had cancelled hundreds of flights to accommodate inspections after Friday's ordeal, when Southwest flight 812, en route from Phoenix to Sacramento, Calif., was diverted to a military base at Yuma, Ariz., after a section of the plane's fuselage ripped open, depressurizing the plane and exposing the sky to passengers.

The five-foot section of the plane's fuselage that opened up has been sent to Washington, D.C., for detailed microscopic analysis.